SAVANNAH - Death penalty appeals from condemned inmates usually hinge on technicalities, legal hiccups and procedural errors. But this week a federal judge will hear evidence from a death row inmate convicted of gunning down a Savannah police officer that will center on a more fundamental question: Is he innocent?

Troy Anthony Davis' attorneys and legal experts alike say such an argument is highly unusual. And many say the hearing, scheduled for Wednesday in U.S. District Court in Savannah, marks the first time the U.S. Supreme Court has ordered such a hearing in a death penalty case.

Then again, just about everything surrounding Davis' case has been extraordinary. He was sentenced to die in 1991 for the shooting death of Savannah police officer Mark MacPhail, who was killed in 1989 while working off-duty at a bus station after he rushed to the aid of a homeless man who had been attacked.

Since then, the case has become a national flashpoint for death penalty critics who argue Davis is the victim of mistaken identity.

Donald E. Wilkes Jr., a University of Georgia law professor and expert on death penalty appeals, said federal courts normally look only at questions of due process and violations of the accused's right to a fair trial. Claims from inmates such as Davis, seeking a fresh look at their guilt or innocence, are routinely rejected by federal judges, he said.

"It's basically a claim of newly discovered evidence. Ordinarily, that's not regarded as a claim of a violation of constitutional rights," Wilkes said. "This is that rare case where it's a death sentence and there's evidence that was not available at trial that they got the wrong guy."

Davis' lawyers say seven witnesses from his trial have either contradicted their testimony or recanted it. Several people have also signed affidavits since the trial saying they heard another man confess to pulling the trigger.

His execution has been delayed three times, partly because of these concerns. In August, the Supreme Court ordered the new hearing that gives Davis the chance to present the testimony that his attorneys hope will prove he was mistakenly identified as MacPhail's killer.

The Davis camp's task, though, is more complicated because two of the seven witnesses have since died. And state courts have discounted as inadmissable any third-party statements that another man confessed to the slaying.

According to court filings, Davis' attorneys plan to call 14 witnesses during the Wednesday hearing. Several are trial witnesses who have recanted all or part of their testimony. Others claim someone other than Davis confessed to the crime.

"Anything other than a very strong, overwhelming case is probably not going to suffice," Wilkes said.

The Supreme Court said the evidence must "clearly establish" Davis' innocence, but what that means remains unclear.

Anne Emanuel, a Georgia State University law professor, said it could mean that Davis must actually prove his innocence, or he could have to prove that a jury could not find enough evidence to prove him guilty - two very different standards.

The result of the hearing could force the Supreme Court to decide whether a death penalty defendant can raise what attorneys call a free-standing claim of innocence - that is, a stand-alone claim that the client is innocent, not that the sentence should be commuted on procedural grounds.

Emanuel said that the High Court has never before ruled that a claim of actual innocence is enough to support a court's decision to commute a defendant's sentence after a prior conviction.

Davis' attorneys have worked for more than a year to convince courts to grant them this hearing, and judges at first balked because much of the evidence his attorneys say could lead to his exoneration was revealed after his conviction.

But the Supreme Court granted the request in August, with Justice John Paul Stevens concluding that "the substantial risk of putting an innocent man to death clearly provides an adequate justification for holding an evidentiary hearing."

Prosecutors, meanwhile, will get another chance to present their side of the argument.

For years they have maintained the case is closed, and they stand by the verdict. They were buoyed by U.S. Supreme Court Justice Antonin Scalia's sharply worded dissent, which called Davis' claim a "sure loser" and noted that state and federal courts have repeatedly tossed his appeals.

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